In short. An unavailability is firm: the calculation always respects it. A preference is a wish: it takes it into account, with no guarantee.
Choose between the two
If you really cannot, declare an unavailability. If you only prefer, declare a preference: preferring nights gives you more of them, and a few day shifts remain normal.
In My availability, a slot of the typical week is preferred or unavailable, never both.
Close or request a specific date
A date is always firm: there is no preference for a single day.
- To not work that day, touch its cell in the calendar.
- To work it, Request a shift, at the bottom of the calendar. Your manager keeps or refuses it. On an unavailable cell, the screen asks you to confirm: this lifts the unavailability for that cell only.